Metal halide high intensity discharge (HID) lamps are desired to run at high wall temperatures in order to improve the efficacy, alter the color temperature, and/or raise the color rendering index of the light source. Typically, the metal halide lamps include fills comprising halides (especially iodides and bromides) of one or more metals, such as Na. Often Na is used in combination with Sc or Sn. Further additions are Th, Tl, In and Li. Other types of filling include rare earth metals such as Tm, Ho and Dy. Lamps which contain such fills have very desirable spectral properties: efficacies above 100 lm/W, color temperatures of about 3700 K, and color rendering indices (CRI) around 85. Because of the low vapor pressure of some of the metal halide additives, the fused quartz lamp envelope must be operated at higher than normal temperatures. At wall temperatures exceeding 900-1000.degree. C., the lifetime of the lamps is limited by the interaction between the metal halides and the wall made from quartz glass. The use of arc tube materials which can be operated at higher temperatures than quartz glass and which are chemically more resistant than quartz glass provides an effective way to increase the lifetime of lamps containing these metal halides.
Polycrystalline alumina (PCA) is a sodium resistant envelope for high pressure sodium lamps. PCA can operate at higher temperatures than quartz glass and it is expected to be chemically more resistant than quartz glass. The PCA vessel is closed at its ends by means of alumina plugs. Gastight sealing is achieved by sealing glass, often referred to as fusible ceramic or frit. However, investigations of metal halide chemistries in PCA envelopes have shown that reactions between the metal halides and conventional frits or even allegedly "halide-resistant" frits severely limit lifetime. An example of such a frit is based on the components CaO, Al.sub.2 O.sub.3, BaO, MgO and B.sub.2 O.sub.3. Consequently, it is highly desirable to find a fritless seal method.
Normally, PCA lamps use feedthroughs made from niobium because their coefficients of thermal expansion are similar. Especially when the fill contains rare earth halides, one problem is involved by the reactions between the Nb feedthroughs and the fill. This problem was alleviated somewhat by using special arrangements wherein the plug and the feedthrough is simultaneously replaced by a plug made from electrically conductive cermets. These cermets are composite sintered bodies usually comprising alumina (the arc tube material) and Mo or W (a conductive halide resistant material).
U.S. Pat. No. 4,354,964, Hing et al., discloses an electrically-conducting alumina-metal (e.g. tungsten or molybdenum) cermet containing 4 to 20 vol. % metal for use as plug members or feedthroughs in PCA (polycrystalline alumina) envelopes of metal halide HID (high-intensity discharge) lamps. The cermet has refractory metal rods (as electrodes or current leads). They are embedded in the cermet body in the green or prefired state and then co-fired during final sintering of the cermet to high density. The method of joining such cermets with PCA tubes is not described. Thermal expansion mismatch between the cermet and PCA, or between the cermet and tungsten or molybdenum electrode can not be eliminated simultaneously. Such differential thermal expansion can result in cracking and leaks in either PCA tubes or cermet, or in both, during lamp on-and-off operation.
U.S. Pat. No. 4,731,561, Izumiya et al., showed one end of the PCA tube was enclosed with a co-sintered electrically-conductive alumina-Mo or W cermet. The other end of the PCA tube was enclosed with a frit-sealed cermet. The cermets were all coated with an insulating layer so as to prevent back-arcing.
U.S. Pat. No. 4,687,969, Kajihara et al, describes besides conducting cermet plugs also non-conducting cermets with feedthroughs passing through and projecting in- and outwardly. One end of the PCA tube has a co-sintered cermet, while the other end has a frit-sealed cermet. However, cracking in the cermet can not be prevented, since the composition of the plug is fixed and is not direction dependent.
All these one-part plugs have the disadvantage that their coefficient of thermal expansion doesn't really fit the surrounding part (e.g. vessel). A solution is suggested for example in U.S. Pat. No. 4,602,956, Partlow et al. It discloses a cermet plug that comprises a core, consisting essentially of 10 to 30 volume percent W or Mo, remainder alumina, and one or more layers of other cermet compositions surrounding the core and being substantially coaxially therewith. The layers consist essentially of from about 5 to 10 volume percent W or Mo, the remainder alumina. Such a cermet plug is hermetically sealed to the end wall of the arc tube by means of "halideresistant" frits.
However, such an electrically conductive cermet plug is not sufficiently gastight over a long period of time.
Another solution is a non-conductive cermet plug having a more dense structure. However, a separate metal feedthrough is needed. U.S. Pat. No. 5,404,078, Bunk et al., discloses a high pressure discharge lamp with a ceramic vessel whose ends are closed by non-conductive cermet plugs consisting for example of alumina and tungsten or molybdenum. In a specific embodiment (FIG. 9) the cermet plug consists of concentric parts with different proportions of tungsten. These parts provide gradually changing coefficients of thermal expansion.
European Patent Application No. 650 184, Nagayama, discusses an arc tube with end plugs consisting of a non-conducting cermet whose features resemble those disclosed in the embodiments of FIG. 1 and 9 of U.S. Pat. No. 5,404,078, Bunk et al. The disc-like plug is made of concentric rings or layers of different composition (radially graded seal). Moreover, in other embodiments (FIG. 16 ff.) the cermet plug is made from axially aligned layers of different composition (axially graded seal). There is a direct sinter connection between the vessel and the neighboring first layer of the plug.
U.S. Pat. No. 4,155,758, Evans et al., discloses in FIG. 14 an axially graded plug, too. However, it is made from three layers of electrically conducting cermet.